1. Field of the Invention
This invention relates generally to a communications array transceiver and, more particularly, to a transceiver for a satellite communications system that employs an array of small, readily transportable antennas that transmit signals that are in phase and aligned in time with each other.
2. Discussion of the Related Art
The military requires robust, reliable and increasingly wideband communications systems to provide for the rapid collection and dissemination of intelligence data and tactical command and control information. There is a great tactical value in providing timely data to, and reports from, mobile units in the field that may be in a hostile environment. Satisfaction of this need requires that communications links be established quickly between the field unit and a remote, sometimes transcontinental, site. It has been recognized that communications by satellite provides the required access in this type of environment.
Modern strategic and tactical communications of this type typically require wide bandwidth communications, for example 40 megabits per second. A certain amount of energy is required for each bit that is to be transmitted. The more bits transmitted per second, the more energy is required per unit time, and thus the more power for the transmission is required. Similarly, a certain amount of energy per bit is required to receive a communication, and wider bandwidth communications require more signal power to be received. The system""s transmission power requirements can be reduced and its receiving power collection capacity can be increased by increasing the antenna gain, which is achieved by increasing the size of the antenna. Therefore, large reception and transmission apertures are usually necessary to supply the gain to handle wide bandwidth signals. For example, to transmit 40 megabits per second in the Ku frequency band, it is desirable to have an antenna that is about 10 meters in diameter.
State of the art satellite communications systems are almost exclusively constructed of a single antenna that has a large aperture and a corresponding large high power amplifier to achieve high sensitivity and high equivalent isotropic radiated power (EIRP) for wide bandwidth communications. Typically, the combination of the large size of the aperture and the amplifier provide a communications system that is unwieldy for rapid deployment in unfriendly terrains. It is possible to transmit the higher data rate signals at lower power by combining identical transmissions from a plurality of smaller, more readily deployable antennas. However, in order to provide such a system, the transmitted bits from each separate antenna must be aligned in time with each other, and the radio frequency carrier transmitted by each antenna must be in phase with each other.
It is known to use phased array antennas to improve sensitivity and EIRP by phasing transmitted and/or received signals. The phased array antennas are typically constructed of a fixed, permanent, rigid physical configuration with closely spaced antenna elements that do not require or implement delay compensations. A variation of this type of antenna is a phased array design that implements xe2x80x9ctrue time delayxe2x80x9d for each element as a means of adjusting the phase of each element. Known designs of this type, however, require and implement delays that have a known relationship from element to element and do not require and do not implement delays that are arbitrary as a result of an arbitrary physical disposition of the elements.
One known commercial satellite communications system that employs more than one antenna is the TACSTAR MK-II, available from Datron/Transco Inc. This system performs phase combining with two independent antenna elements. In this design, the antenna operates only in the receive mode with two closely spaced antenna elements for narrowband signals that do not require delay compensation.
What is needed is a satellite communications system that provides transmission and reception of wideband signals, and includes antennas and corresponding equipment that is easily deployable, rugged, reliable and secure. It is therefore an object of the present invention to provide such a communications system.
In accordance with the teachings of the present invention, a satellite communications system is disclosed that employs an array of separate and easily deployable antennas for transmission and reception purposes to accommodate high data rate transmissions. The antennas can be deployed randomly at a communications site, and are physically separated. Each antenna transmits and receives the same data. A coded signal is used to identify the transmission from each antenna for calibration purposes to align the bits transmitted by each antenna in time and provide phase matching for the carrier wave of each antenna signal. The coded signals are used to compare the phase and timing relationship between each antenna signal and a reference antenna signal when the reference antenna receives all the coded signals for all of the antennas. Correction computations are performed and specialized phase and data alignment systems are employed to delay the various transmitted signals relative to the reference antenna to provide the desired alignment. Additionally, phase and timing systems are used to determine and correct the phase and data timing variations between the data received by the antennas so that they can be combined and processed.